It could be the reason algae was growing considerably faster downstream of the discharge than it was upstream.

The algae was harming pollution-sensitive insect life, such as mayflies, putting the city council in breach of a discharge consent condition that it should not have a significant adverse effect on aquatic life in the river.

The revelation that nitrogen could be the possible cause of the problem has taken the city council by surprise.

Waste and water services manager Rob Green said the results of an agreed, shared monitoring programme had not been completed.

"I was not aware of a nitrogen problem. They have not shared that with us."

If nitrogen had to be removed or reduced, there would have to be a different treatment process used, and a probable retrofit of the plant.

"But I can't pre-empt anything before the results of the study," Mr Green said.

The city council has been carrying out almost constant alum dosing at the plant during the last few months to remove phosphorus, which was the suspected cause of algal growth.

Operating the clarifier to remove phosphorus costs about $3000 a day.

At the end of the programme of water quality tests designed to identify the problem, and suggest a way of fixing it, Horizons is expected to prepare new consent conditions. These would be due by the end of March.

"That's our role as the regulatory authority. To manage the effects we need to impose conditions, even if it costs a huge amount of money," Mr Munneke said.

Before people were invited to make submissions on the consent changes, the city council would be given time to come up with proposed solutions, he said.

The modified submissions process would allow the city council to comment on whether the conditions would work, as meeting them could cost "an enormous amount of money".

Some key interested parties would also be invited to take part in informal consultation before the official submissions process kicked in.

Mr Munneke said the city council understood, "more or less", what the new conditions would look like, but would have until the end of June to respond.

"We don't know how they are going to meet the new conditions."

The full scientific analysis of the monitoring programme is expected to be presented to Horizons' environment committee in March.